There are several situations in which it is useful to know the direction in which a body is moving in three dimensional space other than in a horizontal direction.
One example is in plotting the co-ordinates of a borehole or well.
When boreholes for geological testing and exploration or the like are drilled the hole rarely if ever extends in a straight line from the surface. The drill bit wanders from the straight line path in a relatively gradual but erratic manner and accordingly it is necessary for accurate charting of sub-surface formations and the like to determine the spatial co-ordinates of the boreholes from point to point so that its shape and position may likewise be determined.
Plotting of the coordinates may be achieved by lowering into the borehole a body in the form of a probe which transmits data as to the probe's direction, which is coincident with the borehole axis direction, from positions as the probe advances along the borehole axis and measuring the distance the probe travels between those positions.
A second example would be in plotting the trajectory for example of a rocket when the rocket has a trajectory which is in a horizontal direction for a very short period during the total trajectory.
The present invention is described herein with reference to its application in plotting the coordinates of boreholes but it will be understood that this is merely one application of the invention described and is not limiting.
Boreholes of the type under discussion rarely extend in a horizontal plane and a "borehole" is herein defined as a hole which has no substantial portion extending horizontally.
Normally the measurement of borehole co-ordinates is performed utilizing the polar co-ordinate system with parameters of hole length, slant and azimuth angles. The slant and azimuth angles are functions of the independent variable, namely borehole length, and their values are measured at predetermined points spaced from a borehole origin, which may be the top or bottom of the borehole.
To this end probes have been developed which may be lowered into and withdrawn from the borehole with sensors determining the slant and azimuth angle. It is of course necessary to measure those angles against known reference directions.
For slant angle (or inclination) measurement it is customary to use either the inertial reference of a gyroscope axis or to use the direction of the earth's gravitational field as a reference.
In the case of an inertial reference using using a gyroscope, the slant angle sensing means suffers from the disadvantage of being relatively expensive and delicate and easily damaged by shock. Suitable gyroscope have a limited useful life due to wear. Moreover errors in derived measurements are introduced due to precession and the earths drift rate.
In the case of a gravity reference, the gravity sensing device may be an accelerometer. The use of accelerometers for this purpose has many advantages, including high sensitivity and accuracy, small size, relative robustness and mechanical simplicity. For azimuth angle measurement it is again customary to use the inertial reference of a gyroscope axis, although the same disadvantages apply to this application as apply to their use for slant angle measurement.
The earth's magnetic field has also been used as a reference direction in relation to which to measure azimuth angle at points along a borehole, using for example magnetic gates as sensors, but in that case the instrumentation involved suffers from the disadvantage that error is introduced by localized variations in the earth's magnetic field due for, for example, to the common occurence of iron ores in the borehole vicinity.
Probes carrying gyroscopes and or magnetic field sensing instruments are necessarily of a diameter which is inconveniently large requiring a certain minimum diameter of the probe which prevents it from being used in smaller diameter boreholes which for exploratory purposes at least would be quite satisfactory.
As is well known slant angle may be measured by gravity sensors such as accelerometers in several ways.
For example one accelerometer may be fixed so as to measure the component of the earth's gravitational field in the direction of the probe axis. In this case a signal output from the accelerometer is invariant with rotation of the probe about the borehole axis.
Slant angle may also be measured by two accelerometers set at right angles to each other and to the probe axis.
Generally however it is convenient to use three accelerometers, each set at right angles to the other since in this case the sum of the vectors measured is a constant resulting in simplified calculations, easier error detection, and greater sensitivity over the possible range of slant directions.
Accelerometers have not, however, previously been used to measure azimuth angle.
For an accelerometer fixed to a probe, difficulty in measuring azimuth angle would be introduced by uncontrolled rotation of the probe about the borehole axis.
In that event change in the gravity vector as measured by an accelerometer would need to be resolved into components due to change in azimuth angle, and due to change in inclination, and due to axial rotation. Such resolution is not possible against the single reference direction of the earth's gravitational field.